The present invention relates to a control system for an engine, and more specifically to an electronic control system using a microcomputer in which means for measuring the quantity of intake air supplied to the engine is improved so that a digitally represented measurement output can be provided for effective use and so that the measurement of the intake air quantity can accurately be executed for high-accuracy injection quantity control for the engine even under a high engine load condition.
To electronically control the operating condition of an engine, the engine condition needs to be monitored continually. Monitoring means for the engine condition include means for measuring the quantity of intake air. As an example of an intake air quantity measuring means for an engine, an airflow measuring device of a heat-wire type is conventionally known which is set in an intake passage of the engine. This measuring device is constructed so that a temperature sensing element, which is adapted to generate heat when supplied with a heating current, is disposed in the intake passage. The quantity of air passing through the intake passage is measured by determining the temperature change of the temperature sensing element.
The temperature sensing element is formed of a resistance element which has a temperature characteristic such that resistance depends on temperature. Thus, the temperature of the temperature sensing element can be measured by determining its resistance. Since the temperature sensing element is disposed in the intake passage, the amount of heat radiated from the temperature sensing element varies with the quantity of intake airflow. Therefore, if the heating current, for example, is controlled so that the temperature sensing element is kept at a fixed temperature, the level of the heating current is proportional to the intake airflow quantity. Thus, the intake airflow quantity may be detected from the value of the heating current.
For electronically calculating the injection quantity for the engine to execute fuel injection control on the basis of a measurement signal indicative of the intake air quantity, a microcomputer is used as an arithmetic control means therefor. Thus, the measurement signal from the airflow measuring device is converted into digital data before it is supplied to the microcomputer. Namely, if the airflow measurement signal is analog data such as a current value, the engine control system requires an A/D converter with very high accuracy, thereby complicating system construction.